Amplitude limiting system



PBE 23, i946. F. .J. BINGLEY ET AL 2,399,91

l AMPLITUDE LIMITING SYSTEM Filed Jan. 12, 1942 2 Sheets-Sheet 1 V April 23,1946.

F..J. BINGLEY ETAL 2,399,091

AMPLITUDE LIMITING SYSTEM .24 Sheets-Sheet 2` Fgled Jan, 12, 1942 car-OFF f7/Z or Tagle-26 Patented Apr. 23, 1946 f Frank VVJ. Bingley, Chestnut nil'r'lanafbe'rt c.

, .;MooreVPhiladelpha, `Pa., assignors to Philco l. Radio and Television Corporation, Philadelphia,

Pa.,`acorporati0n. of Delaware i* Application January 1,2, 1942,4seria1vNo. 426,524 (Grasa-20V 11 Claims.

' This'inventionrelates V-toifrequency modulation receivers, and more particularlyto an amplitude limitingsystem for frequencymodulation receiv.'

ers. Y', v 1 i It jhas been common l'practiceto use -some sort of limiting-system in frequency'v modulation receivers'and usually these limiting *systems have required the amplication of the signal to a very hi'g'h value, the'cuttine ofi-of the undesired portion of the signal, andthe amplification ofthe remaining portion. -Y This systemo'f'limitingV has hadthef disadvantage of -wasting a great Adeal vof theamplier gain. 1 -j' f 4 The purposel of limiting devices in frequency modulation receiving' systeiri's is to 'eliminate the eifects' offnoise andstaticvhich appear on the receivedf'car'rier mostly'as 'amplitude modulation. This amplitude modulationcarries no intelligence, and it is, therefore, advantageousr to removefit in the course of amplifying thesignal 1 4 `The Vprincipal object'ofthe present invention is toi provide a novel'method and means for eliminating amplitude 'modulation fromj 'an incoming frequency-modulated carrier wave." i i Another object of the inventionis to provide a lim-itenor amplitude-modulation remover, which does not require the wasting of a great deal vof the gain of the amplifier.

A further object of the invention is to provide a limiter which does notnecessitate 'a separate limiter-tube 1 f y f 1 j 'Still another object ofl theinvention 'is to iprovide a novel method andapparatustvherebyam plitiide modulation is eliminated froma frequen..A cy-modulated carrier wave by first modifying the wave so as tolevel the radio `frequency-signal voltage peaksvonone side of'- thefwavejwhereby amplitude modulation appears onlyonfthe opposite sicle, and then selecting a portion of the modi! fied wave so as to remove the'V amplitude modulation onsaid oppositesi de.`l H f Other objects and features ofthe invention will be apparentto those skilled-in the-artfrom the followingspecication and the *accompanying drawingainwhich: f '1 "2"' Fig. l. is a block diagram of a frequency modus lation receiver embodying the inventionj'g Img fz' is a-circuitdiagram of part ofthe' second,

and fthird intermediate frequencyi 4amplifiers `of thereceiver-shown inFig'L-lyand! f j VFig. 3 illustrates the signal voltage as'l itappeaif's`- in successive jparts'.E of thef receiver, th'e successiveforms ofthe signal .fvolt'age bei'ng shovvniWitlirey spect to a time axis vt at' zero signal or-grou`iic'i.-'level.` ,lteferringrstto Fig. 1, 'there is'shown a fre# fluency-modulation receiverhaving'iafsuitable dipolearitenna I connected bya transmission lline 2 to `a Ymatching transformer' 3; 'which -preferably supplies the receivedfrequency-modulated' carrier signal l'to al radio frequency stage si; 1 rljhis stage 4 may 'consist o flone or'several'vacuum tube amplifiers. 1`I:I:`"desired, however, the stage!! may be omitted entirely. V The'olitput of the radio fre.I quency stage coupled to the tuned converter stage 5 into which isvalsol injected-'the outputof the oscillator 6, 'which may be tuned by7 a section of a variable gang condenser 'soa'stofcause a desired beat or intermediate Vfrequency in the-"con-V verter, asis well known'fin superheterodyne re-` ceivers. In the case of frequency-modulation vreceivers, this'intermedia'te frequency is often estab- The carrier wave is amplified in several stages at intermediate frequency, andin the present case there fare" shown fthree intermediate amplifier' stages, vthe iirst` of which, *1, is 'an intermediate amplier [stage of `conventional desigmand the secondand third of which, 8 and 9, are specially designed stages embodyingthe present invention:v The output of the third 'intermediate' 'frequency stage is supplied to the `second'detector IU Whichmay be advantageously ofthe balanced" type', to cause a furtherl reduction of noise.

` The detected' signal is then applied to the laudio frequency amplifier] I', which in turn isconnected to'the sound'translating -device' I 2`, e. g., a lcud' speaker, vtranscription inachine, lor#` other device.

In Fig-2 there-isshowna circuit diagram of the second andithirdi'ntermediat'e frequencyla'm# pliers illustrating the `preferred embodiment Vof the invention.- The first intermediate frequency 23.. Afrise 'pentode-rtube is; is` nesecitol near' eutoff in 'anysui'tabIe manner andconsequently, in addition to its operation as'anfamplifier,italso functionsas "a biased "detect'or j tube vvitli` [regard quency-wavef Thesignal,v voltage appliedfto' the control grid istituite I3 is shown at am Fig.

3, while the plate current of the tube is shown at C. The displacement of the axis of signal voltage B from the axis t corresponds to the bias voltage. As a result of the plate current C, there will appear on the anode of the pentode I3 an amplied voltage, e. g., as shown in Fig. 3 at D, containing the saine amplitude and frequency-modulation as is present on the voltage wave B." .The.;displacexnent of signal voltage D from'axis t corresponds to the D. C. voltage of the plate supply. Now since the tube I3 is acting as a biased detector, there will also appear across the resistance .22.-in the-load circuit a voltage which is proportional to theamplitude-modulation envelope of the :signal applied to the control grid. 'Ihis voltage r'is that'sh'own the intermediate frequency, -noiappreciablerinten mediate frequency component will appear across the resistor 22. plate circuit -of the ltube I3 .higher vharmonics of the intermediatev frequency, but.'since'ihere isgno loading impedance resonant .to itheserharmonics, they will. produce l.no voltage -ixrsucoeeding stages.

AAppearing across-the .fsecondaiycoil I9 of lthe I. F. transformer I1 will-.be.avoltage.wave having .the Acharacteristics of thevoltajge LWalle impressed on the control grid-of 4tubej.v Appearingfzacross the resistor .24, whichis .coupleditoresistor 22 by means of the couplingcondenser 25, .-wl1l:be.a voltage .wave vhaving the characteristics tof the amplitude-.modulation of .the :original wave. Since l.coil Ill-and resistorf24 .are .infseries with respect to the input -circuitof :the third .1. F. lamplier tube 26, there beappliedrto'the control Kgrid 30 ,ofthistubea Acompositesig-nal voltage which is equal to the sum of the intermediate '-frequency :signal voltage across the :secondary winding IES .and the .audio frequency v.signal voltage across the .resistorflt .Such composite signal voltage ,is shown V,at ,F 3. .Theeiect of combining the I. F. and raudio signal voltages (D and E) is to level fthepositiveLLF. 'Signal .voltage peaks, whereby amplitude-modulation .appearsonly on the negative-:side of the carrier-wave. By-aproperselection ,of the impedance-characteristics of thel., F. andA. F. portions. of fthe circuit, the relative values of ,these `two voltages and ,the

that .the positive peaks '.of the .1. 3F. wave .are brought to a common level :and .hence "show no 'effects of :amplitude-modulation, all :of the :am-k

plitude-modulation Vbeing on @negative iside of .the I. F. voltage wave. Thesignal voltagafas applied to control .grid SII of tube-#Maud vwith a. D C. bias component added,jis shown .at Gun Eig.

The term peaklevelling as usedin various formsV in this specification, maybe defined'asthat operation vupon an amplitude :modulated :wave which consists in reiormingthe wave aboutg-its zero axis :in a manner .-such that all. F. voltage peaks on one sideof the wave 4reachgacommon absolute voltage level. A levelled wave characterized in that its zero `axis. is effectively .fdistortecl .in .such a way that-all .of its amplitude modulationis transferred to thatr side of .the wave whose R. F. Avoltagepeaksare not levelled. ,Levelling is not `to Ahe confused with the :roperation knownas limiting which isaccomplishedby ,cutting off thewavepcaks.

.In order to ,eliminate .theeiects of the .-ampli.A

tuile-modulation appearing on .the ,negative side of the-.voltage .waveGathe tube 26fis biasedbelow itscut-.oii value which isiridicatedpy-the broken There will also appear Vin the amount that these tops fail to reach the line c, so that only the positive part of the voltage wave is effective to cause output current in the plate circuit of the tube. By properly selecting the value of bias on the grid of the tube 26, the amplitude-modulation can be entirely cut oi without materially aiecting the amplication of the stage. The plate current of the tube 26 will be asindicated at H in Fig. 13, and the I. lF. signal voltage appearing acrossthe tube output or load impedance 21 will be without amplitude-modulaftion, as is shown at I in Fig. 3.

`:'IfhuS., it willbe seen that amplitude-modulation `of a 'frequency-modulated carrier Wave is eliminated :according tothe present invention by first modifyingfthe wave so as to level the carrier sig- :nal'lvoltagejpeakson one side of the wave, thereby :eliminating amplitude changes on that side. zand Ytheir selecting :a'portion of the modified wave lso as Atn reject the amplitude-modulation on the other side. Moreover, this is done by deriving from the wave a jhigh :frequency .signal-.voltage and-a:low'frequency-signal voltage, the latter'correspondingto the amplitude-modulation frequency, and then combining the two derived signal voltages to obtain the desired modified signal.

There available `the output circuit of the tube I3 anamount `oi .total power whch ,.has.a denite `nite Vmaximum value, 4and the division of power-between the I. lib-and F. ,portions .of'theload ci-rcuitof said tube will be determined bythe relative limpedances, of these twoportions. Sinceit is desired-to balance'cut .the amplitude changes on one side of the carrier-wave,:it;is necessary-to carefully .select these impedance values so as to obtain `:the -balancingaction It visallossible `.to -design a radio `frequency .circuit` tojgive a phaseshift .to lcorrespond .to .any .audio ffre quency Acircuit vso that if i the vil. F. circuit is fdesigned rst, the fR. F. circuit maybe readily de-y signed to correspond.

, Itis importantjn the desi-gn of thissystemthat the. phase ,of .the amplitude-modulation .Wave shown'at E, inF-ig. 3.be.inphase with the ampli tuile-modulation envelope von ythe intermediate frequency waveshown-at D. The moreexactthis phase coincidence, kthernore effective will be the limiter. Y

.In Fig. 3 of the drawings itisto beunderstood that the .relative amplitudes auf the wavesinre .not intended to :be illustrated. actual practice in the various v,parts of :the circuit, for .-example,

the waveshownatD .in :Eilat .wouldfbe many times ythe amplitude of the wavefsliown-:at- Bdn Fie..3. a

`It will .be found necessary to supplya l.certain minimum signal Astrength to the slimiter togget eiective operation, vfor unless :the composite .sig-V nal .voltage is .of suicient strength to-make'the positivewave tops uniform Tand themegative wave tops always reach the cut--oioV point oftheqtube, some .amplitude-modulation will come through, and the amount of;amplitudemodulation which does come through will be proportional -fto the Cut-.Dff value. l

While vthe limiting system kof the invention :has lbeen illustratedand described aslused with 4variablecondenser tuning.means,it isintended ltohe Y used with any tuninglmeanaWhethercapactiVe or inductive, continuously` variable or otherwise. Moreover, the 4 particular. :circuit ,illustrated .and describeidzis not to herconsidered as limiting of. the invention, y.fcrxthere fare'numerous ;mo'dicatlons which. mghthe made by .une skilled .inthe :radio assegnarvart to vary the specific form ofthe inventicm Without departing from'its'scope las deiined'by-Lthe appended claims. Y1 f ,i Wenlaimz ,v f. fe f A I' 'Y m-.L'A method ofreducing amplitudevariations ofra vfre'quencil-modulated `carrier vwave,'..vvhich paratus for reducing amplitude variations of'a received carrier wave,"comprising means for deriving vfroms'aid Wave a signal voltage having the same frequency characteristics and including said amplitude variations, means for deriving from said wave a second signal voltage corresponding to the envelope of said Wave, means for combining said derived signal voltages soV as to produce a modiiied signal voltage in which the peaks on one side are substantially leveled, and means for selecting a portion of said modified signal voltage substantially free of amplitude variations.

3. In a radio receiver adapted for the reception of frequency-modulated carrier waves, apparatus for reducing amplitude variations of a received carrier Wave, comprising means for deriving from said Wave a signal voltage having the same frequency characteristics and including said amplitude variations, means for rectifying a portion of said wave to derive a second signal voltage corresponding to the envelope of said wave, means for combining said derived signal voltages so as to produce a modied signal voltage in which the peaks on one side are substantially leveled, and means for selecting a portion of said modified signal voltage substantially free of amplitude variations.

4. In a radio receiver adapted for the reception of frequency-modulated carrier Waves, apparatus for reducing the effects of undesired amplitude-modulation on a received carrier wave, comprising an electron discharge tube having an output circuit and arranged to operate both as an amplifier and as a detector, a first load impedance in the output of said tube having relatively high impedance at radio frequency and relatively low impedance at lower frequencies, a second load impedance in the output of said tube for developing a voltage corresponding to the envelope of said carrier Wave, whereby a resultant signal wave is produced, and means for transmitting a portion only of said resultant wave substantially free of amplitude Variations.

5. In a radio receiver, a source of frequencymodulated carrier Wave signal, a vacuum tube, means for supplying said signal to said vacuum tube, means for biasing said tube so that it operates both as an amplifier and as a detector, thereby to produce a variation in plate current which is proportional to the low-frequency amplitude-modulation component on the carrier Wave, a resonant load circuit responsive to the carrier-frequency output of said vacuum tube,

a non-resonant load circuit responsive to the low frequency variations in said plate current, a second vacuum tube amplifier having an input `circuit;fmean's'forfhiasing said' second vtubesub'- stantially tolcutz-"oif,` andfc'onnections .for-y applying the "sum of. theFsignal'f-voltages 4 across said -loa'dtcircuits to :the 'input circuit of said second vacuum-tube. 1 l, if 1 -6i A^`systenr for limiting a radio'frequency Wave 1 to i substantially remove amplitude-modulation therefrom, comprising meansy for y.amplifyingthewave, means for rectifying a portion of the wave, means for isuperimposing. the rectified component onrisaid amplified vradio frequency Wave,1the phase and amplitude of said rectified component beingvsu'ch as to lvelthe positive peaksof' 'thekradio frequency Wave; means' for lremoving the negative portion of said Wave; and means for .further amplifying thepositive' portion of said Wave, I i l 7 In an amplitude-limiting circuit, a-source of'high frequency alternating current signal voltage, means for establishing -a lowfrequency' signal voltage Whose* wave .form *correspondsi sub'- stantially tothe envelope of said high; frequency signal voltage, means for combining said signal voltages in such relative lproportions that the positive signal voltage peaks of the high frequency signal voltage all reach substantially the same absolute voltage, and means responsive to said positive peaks for establishing a new high frequency alternating current signal voltage of substantially constant amplitude.

8. The method of deriving a frequency-modulated high frequency signal wave substantially free of amplitude modulation from an incoming signal wave which is both frequency-modulated and amplitude-modulated, whichr method consists, first, in detecting said incoming signal Wave to derive therefrom a low frequency Wave corresponding to the amplitude modulation of said incoming Wave; secondly, combining the signal wave and the derived low frequency wave With the latter substantially in phase with the envelope of the negative side of said signal wave, and; thirdly, limiting the amplitude of the resultant signal wave to such extent as substantially to cut off amplitude modulation on the negative side only of said resultant signal wave.

9. The method of deriving a frequency-modulated high frequency signal wave substantially free of amplitude modulation from an incoming signal Wave which is both frequency-modulated and amplitude-modulated, which method consists, first, in detecting said incoming signal wave to derive therefrom a low frequency Wave corresponding to the amplitude modulation of said incoming Wave; secondly, impressing the signal Wave and the derived low frequency Wave, concurrently, upon the control electrode of a vacuum tube amplifier, with said low frequency Wave substantially in phase with the envelope of the negative side of said signal Wave, and; thirdly, limiting the amplitude of the resultant signal wave in the plate circuit of said amplifier to such extent as substantially to cut off amplitude modulation on the negative side only of said resultant signal wave.

l0. In a signal receiving system, a source of high frequency incoming waves having both frequency modulation and amplitude modulation,

a detector coupled to said source and operative i to derive therefrom a low frequency Wave correspondingto said amplitude modulation, a vacum tube amplier having a controlv electrode coupled to said detector tc receive therefrom a low frequency voltage corresponding to said derived 10W frequency wave.'a plate circuit for said f amplitler, "said :control electrode. ,being'ecoupledI-'to said :source to sreceive, concurrently with :said

`llow frequency 'volta-ge, flafhigh 'ffrequency lfv'olta'ge 'corresponding to isaid incoming -fwavx=:,` Y the -`rtwo voltages thus impressed upon said `eontrolfelec 'trade lloeingso :co-'related that the low frequency voltage-is inphase with the -'negative side of the envelope of the high frequency.voltage -and-'sub: stantially equal inamplitude Ato the co-iuciden't amplitude variation "of said vnegative side, Vand means :negatively biasing said 'control 'electrode to such 'extent that Ithe negative Yside fof the resultant high Yfrequency 'Wave in the plate circuit of'said ampliiier-isllimited-inaamplitudesu- -ficiently to exclude atleast the -gleater-pamt 'of the amplitude .modulation therefrom. Y

11. In a signa-1 receiving system, a first vacuum ltube amplifier biased to .function ias-adetector and amplifier, 'a .-sourceof `imzzon'iin'g signal waves having both frequency modulationand anfiplit'ucle modulation, the' input lside of said first amplifier Abeing coupled to said source, 'a

- ampli er.

second vacuum tube -famplierhaving- 'uscenti-o1 ielectrod'e fand a .plate circuit, feoupling :means between the output side of sald'frstfamplier and the control electrode of said second @amplierfor transmitting to :'saii A'contini zelectrodean -amplied r'voltage corresponding 'to fsaid envelope `of the negative side of `thefsigncl Wave Voltage .impressed upon said controlselectrode `:and `meazns negatively @biasing fsa'id control relectrode suiciently .to cut Aoil amplitude modula- -tion from the negative v'-sid'e'of the i-signal `Wave appearing in 4the plate circuit lof said second .3. BINGLEY. i ROBERT c. MOORE. 

